The watt (symbol: W) is the SI derived unit of power.

## Definition

One watt is one joule of energy per second.

1 W = 1 J/s = 1 newton meter per second = 1 kg·m2·s−3

## Origin

The watt is named after James Watt  for his contributions to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th Conférence Générale des Poids et Mesures  in 1960.

## SI multiples

Multiple Name Symbol Multiple Name Symbol
100 watt W
10¹ decawatt daW 10−1 deciwatts dW
10² hectowatts hW 10−2 centiwatts cW
103 kilowatts kW 10−3 milliwatts mW
106 megawatts MW 10−6 microwatts µW
109 gigawatts GW 10−9 nanowatts nW
1012 terawatts TW 10 −12 picowatts pW
1015 petawatts PW 10−15 femtowatts fW
1018 exawatts EW 10−18 attowatts aW
1021 zettawatts ZW 10−21 zeptosiemens zW
1024 yottawatts YW 1024 yoctowatts yW

## Derived and qualified units for power distribution

A watt is a unit of power or the amount of energy per unit time.

### Kilowatt-hour

When paired with a unit of time the term watt is used for expressing energy consumption. For example, a kilowatt hour, is the amount of energy expended by a one kilowatt device over the course of one hour; it equals 3.6 megajoules (1 hour = 3600 seconds). These units are often used in the context of power plants and home energy bills.

For the use of watts as a measurement of transmitter power in radio, see effective radiated power and nominal power.

### MWe, MWt

Watt electrical (abbreviation: We) is a term that refers to power produced as electricity. SI prefixes can be used, for example megawatt electrical (MWe) and gigawatt electrical (GWe).

Watt thermal (abbreviation: Wt). This is a term that refers to thermal power produced. SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt). For example, a nuclear power plant might use a fission reactor to generate heat (thermal output) which creates steam to drive a turbine to generate electricity.

## Other information

This SI unit is named after James Watt . As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (W). But when an SI unit is spelled out, it should always be written in lowercase (watt), unless it begins a sentence or is the name "degrees Celsius".